Passive Tone Controls

Different capacitors change the sound by lowering or raising the cut-off frequency. Moving to
higher values lowers the cut-off frequency and vice versa.

In last month’s column [“Cheap and Easy
Bass Mods,” May 2012], we began our
bass-modding adventures by looking at
ways to wire and configure passive pickups
and potentiometers. Let’s stay with the
“cheap and easy” theme this month and
explore passive tone controls.

Most basses come with a treble roll-off
knob. Essentially, its job is to reduce the
highs in a full-range signal and simulate a
dampened upright bass or darker flatwound
sound. I’m not sure how many bassists
actually use this control. Unlike guitar, treble
isn’t the most important tonal range in
a bass’ frequency spectrum, so perhaps these
tone controls are on basses simply because
guitars offer them and designers feel obliged
to share the love.

That said, the standard tone knob resides
on many basses. If this includes yours, you
should either make good use of it or swap it
for something better. (We’ll explore the latter
in a future column.)

Meanwhile, let’s review the situation: Our
signal chain starts with the pickups and their
specific sonic signature. A pickup’s characteristic
peak is its resonant frequency. When
we use our tone controls, we essentially
modify the size and position of this peak.
As with all passive systems, a tone control
can only reduce a part of the spectrum, but
never add to it. Boosting a frequency is an
exclusive feature of active electronics.

Unlike an active EQ—which can cut a
specific frequency and even some surrounding
ones—a passive tone control cuts only
higher frequencies. The treble knob is a simple
(or first degree) filter that’s formed by a
resistor and a capacitor shunt to ground. Its
cut-off frequency is mainly dictated by the
capacitor’s value, at least for a given pickup
and potentiometer combination.

A very common value is 47 nF. If you
want to experiment with capacitor value,
get a variety of different values (20 nF to
100 nF are usable values to start with)
and test them out with your tone control.
Moving to higher values lowers the cut-off
frequency and vice versa.

Here’s a tip: Instead of soldering and
unsoldering each capacitor to your potentiometer,
solder two wires to the pot and
then lead them outside the control cavity
for easy access. You can then solder some
clips to these wires or simply use bare wire
to attach each capacitor. This trick allows
you to easily audition the capacitors one
after another.

You can see in the diagram how changing
capacitor value is a pretty limited way
to modify your bass guitar tone. Not that
it doesn’t have much effect—it does—but
we aren’t changing anything in the lower
regions that characterize the bass spectrum.

The only way to do this and still stay
passive is to use L-C filtering. An L-C
filter is basically a network of capacitor,
resistor, and inductor. Depending on the
values and wiring, you can put a notch
in your spectrum and vary its position,
width, and depth. Instead of calculating
the values and getting the parts on your
own, I recommend looking for a commercial
solution. Such L-C filters can come
with a simple pot for a single frequency
or rotary switches that directly dial in
various presets.

One problem is that in a passive circuit,
the parts interact with each other and it
can get rather complicated to determine
the sonic outcome. In practice, this means
that an L-C filter’s tonal shaping will shift
when you add in a second pickup. Though
complex, the technical background is very
interesting, and if you want to dig deeper,
you can easily find more info on the web.

But does anyone use L-C filters? I’ve
rarely had a bass with L-C-filtering on my
workbench or seen it anywhere out in the
wild. For me, L-C filtering is too variable
and doesn’t provide enough visual information
to be useful, especially onstage when
you need to act fast.

The passive tone pot can tame an aggressive
sound or let you quickly adjust to the
sonic demands of different playing styles,
and it’s always right at your fingertips. But
other than that, you can do more effective
sonic shaping by working with controls on
your amp, tweaking your pickups, or using
an active tone control.

Before you label me as someone who
always cranks everything wide open, I’ll
leave you with a short teaser for the next
column: Where is your place in the mix
and what strategy gets you there?

Heiko Hoepfinger is a German
physicist and long-time bassist, classical
guitarist, and motorcycle enthusiast. His
work on fuel cells for the European orbital
glider Hermes got him deeply into modern
materials and physical acoustics, and
led him to form BassLab (basslab.de)—a
manufacturer of monocoque guitars and basses. You can
reach him at
chefchen@basslab.de..

Heiko Hoepfinger is a German physicist and long-time bassist, classical guitarist, and motorcycle enthusiast. His work on fuel cells for the European orbital glider Hermes led him to form BassLab (basslab.de)—a manufacturer of monocoque guitars and basses.

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